@maver - sideload not being an option, when you unpack the FP2 dev-channel zip, there is a ./flash_FP2_factory.sh script who bundles all fastboot commands that need to run. I think they went with that approach to include all firmware files (modem, etc…).
Another user noticed the docs being off for the dev builds at Fairphone - FP2 - FP2 - Documentation Suggestions - #22 by HippiE